Joe Gutheinz retired more than a decade ago from his job as a senior agent for the National Aeronautics and Space Administration’s office of inspector general.

But the Houston attorney hasn’t had any trouble keeping busy. In addition to teaching a college class on investigative techniques, he devotes his time to tracking down missing moon rocks, with the help of his students, the Christian Science Monitor reports.

So far, they’re recovered 79. But 160 of the 270 samples collected by American astronauts on the moon between 1969 and 1972 and given to other nations as gifts are lost, stolen or otherwise unaccounted for, Gutheinz says, not to mention 18 more that U.S. states can’t locate.

Over the past 40 years or so, some of the the humble-looking objects have been stored or even discarded. Others purportedly have been put up for auction at hefty prices.

The moon rock given to Arkansas has been found amongst former Gov. Bill Clinton’s belongings. But there’s no sign of the one that went to now-deceased Libyan dictator Moammar Gadhafi.

While some try to hide their prized piece of the lunar surface or have fought court battles to try to retain ownership, others are willing to simply hand over a moon rock to the proper authorities.

“From time to time, I get a call from somebody that has a moon rock and his father or her father died and was a scientist,” Gutheinz told the Monitor. “And they ask, ‘What do I do with it?’ I tell them, ‘Give it back to NASA.’ “